1. Field of the Invention
The present invention relates to a centrifugal fan, and more particularly to a centrifugal fan which is reduced with a thickness and a noise.
2. Description of the Related Art
In the related art, as a fan widely used for cooling, ventilating, air conditioning of household electrical appliances, OA (Office Automation) equipment, and industrial equipment, air conditioning for vehicles, blowing, etc., a centrifugal fan has been known. As a related-art centrifugal fan, a centrifugal fan in which a casing is configured by an upper casing and a lower casing, an impeller is housed between the upper casing and the lower casing, and air suctioned from an inlet with rotation of the impeller is discharged from an outlet formed in a lateral surface between the upper casing and the lower casing toward the outside has been known (for example, see JP-A-2012-207600).
FIG. 5 illustrates a centrifugal fan 100 described in JP-A-2012-207600, where a quadrangular casing 120 is configured by an upper casing 121 and a lower casing 122, and an impeller 130 is housed between the upper casing 121 and the lower casing 122. The impeller 130 is provided with an annular shroud 131. The annular shroud 131 is configured to have a cylindrical part 132 in the center thereof and to be warped toward the upper casing 121 from the cylindrical part 132 to a periphery of the annular shroud 131, and is configured such that a shape of an upper surface from an end of the cylindrical part 132 of the annular shroud 131 to the periphery of the annular shroud 131 is formed in a shape of a curved surface in which four circular arcs having different radii of curvature are connected, and a cross-sectional area of an air flow passage up to the periphery of the annular shroud 131 is gradually increased. Thus, air is increased in pressure, and blows out from a periphery of the impeller 130 to the outside.
Air suctioned from an inlet 110 by high-speed rotation of the impeller 130 passes between the blades 135, blows out from the periphery of the impeller 130 to the outside, and is discharged from an outlet 111, which is formed in a lateral surface between the upper casing 121 and the lower casing 122. However, since the centrifugal fan 100 described in JP-A-2012-207600 has the shape in which the periphery of the annular shroud 131 is warped toward the upper casing 121, part of the air blowing out from the periphery of the impeller 130 may flow back from a gap 125 between the annular shroud 131 and the upper casing 121 toward the inlet 110, and a disturbance may occur at a flow of the air in the vicinity of the inlet 110 due to the air flowing backward, which is responsible for a noise.
When the full height the centrifugal fan is designated from restriction of a space mounted on the equipment or apparatus, the centrifugal fan 100 described in JP-A-2012-207600 may not meet a demand for reduction in thickness due to a height dimension of electronic components such as control IC, etc. mounted on a circuit board 140.